Editing Energy (section)

== Units of measure ==
[[File:Joule's Apparatus (Harper's Scan).png|thumb|right|Joule's apparatus for measuring the mechanical equivalent of heat. A descending weight attached to a string causes a paddle immersed in water to rotate.]]
{{Main|Units of energy}}

In the [[International System of Units]] (SI), the unit of energy is the [[joule]]. It is a [[SI derived unit|derived unit]] that is equal to the energy expended, or [[Work (physics)|work]] done, in applying a force of one [[Newton (unit)|newton]] through a distance of one metre. However energy can also be expressed in many other units not part of the SI, such as [[erg]]s, [[calorie]]s, [[British thermal unit]]s, [[kilowatt-hour]]s and [[kilocalorie]]s, which require a conversion factor when expressed in SI units.

The SI unit of [[Power (physics)|power]], defined as energy per unit of time, is the [[watt]], which is a joule per second. Thus, one joule is one watt-second, and 3600 joules equal one watt-hour. The [[centimetre gram second system of units|CGS]] energy unit is the [[erg]] and the [[imperial and US customary measurement systems|imperial and US customary]] unit is the [[foot pound]]. Other energy units such as the [[electronvolt]], [[food calorie]] or thermodynamic [[kilocalorie|kcal]] (based on the temperature change of water in a heating process), and [[British thermal unit|BTU]] are used in specific areas of science and commerce.

In 1843, [[English people|English]] [[physicist]] [[James Prescott Joule]], [[namesake]] of the unit of measure, discovered that the [[gravity|gravitational]] [[potential energy]] lost by a descending weight attached via a string was equal to the [[internal energy]] gained by the water through [[friction]] with the paddle.